The reprocessing of spent nuclear fuel assemblies has made large amounts of plutonium available.
Using this plutonium in order to mix it with uranium and thereby form nuclear fuels has been suggested for a long time. These fuels, which contain, before irradiation, a mixture of uranium and plutonium oxides are generally called MOX (Mixed Oxide) fuels.
The assemblies containing such MOX fuels, called MOX assemblies in the following, have been loaded into the cores of nuclear reactors where they coexist with assemblies, the nuclear fuel of which before irradiation does not contain any plutonium oxide but only uranium oxide. Such assemblies will be called UO2 assemblies in the following, and the fuel which they contain UO2 fuel. A nuclear reactor core loaded with assemblies of MOX fuel and with assemblies of UO2 fuel will be called a mixed core in the following.
The isotopes of plutonium and those of uranium have very different neutron properties and in particular differences in cross section.
Considering that these differences in neutron properties made it impossible to purely and simply substitute UO2 fuel with MOX fuel in order to produce MOX assemblies, document FR-2 693 023 described a zoned MOX assembly, i.e. for which the nuclear fuel rods have a same plutonium isotope composition (or vector), i.e. the same composition in terms of percentages of the respective mass fractions of each isotope making up the plutonium, and nominal total plutonium mass contents different from one zone to the other of the assembly.
Thus, the nominal total plutonium mass content is lower on the faces than at the centre of the assembly, and even lower in the corners of the assembly. With this, it is possible to obtain a radial distribution of the linear power density in the core of the nuclear reactor, in particular in the peripheral rods of the MOX assemblies adjacent to UO2 assemblies, which is acceptable.
Moreover, as recalled in this document, the plutonium stemming from reprocessing has an isotope composition which strongly varies in particular depending on the initial uranium 235 enrichment, on the burn-up rate and on the storage duration of the fuel before reprocessing.
In order to compensate for the neutron behavior differences which such differences in isotope compositions might induce, energy equivalence relationships were established in order to determine the nominal total plutonium mass contents for different isotope compositions corresponding to a reference uranium 235 content. With these equivalent nominal total mass contents it is possible to compensate for the differences in isotope compositions and to reach the same burn-up rates in the same type of fuel management. The equivalence relationships use equivalence coefficients which depend on the isotope composition of the relevant MOX fuel, i.e. on the isotope composition of the plutonium and the uranium 235 content of the uranium associated with the plutonium.
These equivalence relationships are for example mentioned in pages 41 to 43 of the document entitled Status and Advances in MOX Fuel Technology, Technical Reports Series No. 415 and published by the International Atomic Energy Agency in 2003.
As an example, the table below specifies in its first portion the typical compositions of the plutonium stemming from the reprocessing of UO2 fuel assemblies for a pressurized water reactor as a function of the initial uranium 235 enrichment of the fuel and of the burn-up rate attained by the fuel, the storage duration before reprocessing being the same for all the examples mentioned in the table.
The table specifies in its second portion (last line) the nominal total plutonium mass contents with which it is possible to attain the same burn-up rate as a UO2 assembly enriched to 3.70% by mass of uranium 235 and therefore to compensate for the decrease in the quality of the plutonium: reduction in the amount of fissile isotopes (plutonium 239 and plutonium 241) and increase in the amount of absorbent fertile isotopes (plutonium 238, plutonium 240, plutonium 242 and americium 241).
UO2 assembly for a pressurized water reactorEnriched Enriched Enriched Enriched to 3.25% to 3.70% to 4.00% to 4.95% by massby massby massby massof 235U andof 235U andof 235U andof 235U andirradiated irradiated irradiated irradiated Origin of the at 30 at 40 at 50 at 70 Pu usedGWd/tHMGWd/tHMGWd/tHMGWd/tHMIsotope 238Pu1.22.13.15.3composition239Pu62.257.753.648.9(% by240Pu23.024.124.924.9mass)241Pu8.08.79.19.3242Pu4.46.17.910.2241Am1.21.31.41.4Nominal total Pu6.77.68.710.7mass content (%)equivalent to anenrichment to3.70% by mass of235U
Taking into account the more and more substantial available amounts of plutonium, certain producers of electricity have desired that the newly built nuclear reactors may be loaded with up to 50% of MOX assemblies.
Document U.S. Pat. No. 6,233,302 describes a nuclear reactor in which all the nuclear fuel assemblies loaded into the core contain MOX fuel. In order to ensure a homogeneous radial distribution of the linear power density, these assemblies always have a zoned configuration and further comprise nuclear fuel rods which do not contain any plutonium oxide and for which the nuclear fuel in addition to the inevitable impurities resulting from the manufacturing, contains consumable neutron poisons such as erbium oxide.
However, this reactor does not allow optimum use of the plutonium and these assemblies are complex and costly to produce.